1. Field of the Invention
The present invention relates to a high-frequency electric wire, a manufacturing method thereof, and a wire harness.
2. Description of Related Art
In the related art, a litz wire used to transmit high-frequency signals is known. The litz wire includes a conductor obtained by twisting together multiple wire strands, each of which is obtained by coating a metal conductor with an insulating layer. Typically, high-frequency signals are known to flow only through a vicinity of a surface of a conductor due to a skin effect during transmission of the high-frequency signals. Since the litz wire includes the conductor formed of the multiple wire strands, high-frequency signals flow through the surface of the metal conductor of each of the wire strands. As a result, it is possible to suppress an increase in high frequency resistance due to the skin effect.
In such a litz wire, for example, a minimum diameter of the wire strand is approximately 50 μm. When a depth of the skin during transmission of high-frequency signals is less than 50 μm, a center side of the metal conductor becomes a waste metal portion through which current does not flow except for a surface side of the metal conductor.
A high-frequency electric wire, in which a metal-pipe wire rod with a shape of a circular cylinder is used instead of the wire strand, has been proposed. Since the metal-pipe wire rod is used in this electric wire, a portion corresponding to the waste portion is hollow, and thus it is possible to reduce the costs of the wire rod (refer to Patent Document 1: JP-A-2011-124129).
Patent Document 1: JP-A-2011-124129
However, since the metal-pipe wire rod is used in the high-frequency electric wire disclosed in Patent Document 1, it becomes difficult to reduce a diameter of the high-frequency electric wire. That is, in the metal-pipe wire rod, a metal portion with a thickness of approximately 10 μm is required to be formed on the outside of a hollow portion with a diameter of 50 μm so as to transmit high-frequency signals equivalent to a wire strand with a diameter of 50 μm. Accordingly, the diameter of the metal-pipe wire rod becomes 70 μm, leading to a hindrance to the reduction in the diameter of a high-frequency electric wire.
Since each of the wire strands or the metal-pipe wire rods has a circular outer circumference, gaps therebetween are formed, even after twisting. When wear properties of a sheath material are taken into consideration, a finished outer diameter of the high-frequency electric wire is limited.
When the conductor obtained by twisting together the wire strands is used, as a countermeasure for such a problem, the gaps are eliminated, and the diameter of the high-frequency electric wire is reduced by compressing the conductor formed of the multiple wire strands. However, when the metal-pipe wire rod is used, the hollow portion is blocked by compression, and thus a similar countermeasure cannot be adopted.
As such, in the related art, it is difficult to reduce the outer diameter of the electric wire while preventing an increase in high frequency resistance caused by the skin effect, and while reducing costs.